1. Field of the Invention
This invention relates to an image compression apparatus, and more particularly to an image compression apparatus that achieves compression of digital color document image data read through an image reader with a high compression ratio without sacrificing the visibility of objects such as characters.
2. Description of the Related Art
An enormous amount of data is generated when an original image optically read by an image reader such as a scanner is digitized for used as document image data. The amount of data produced without compression can be so large that it poses significant problems when the data is stored or transmitted over a network.
For example, the size of image data in an A4-sized image with 24 bits per pixel of color information read at a resolution of 300 dpi may be as huge as 24 megabytes.
Conventionally, such huge raw image data has been compressed with compression technologies such as JPEG compression to several hundred bytes of image data.
However, JPEG compression and other compression technologies have a problem that when color document image data containing characters is compressed and then decompressed to reconstruct the original image, the visibility of the characters displayed is decreased.
Further, when image data is compressed with MMR compression in order to prevent the visibility of characters from degrading, the compression ratio of photograph data contained in the image is also decreased.
To solve these problems, apparatuses (refer to Japanese Patent Laid-Open No. 2003-018413, for example) have been devised that separate a region containing characters from an area containing a photograph in a color document image and different compression technologies suitable for the different regions are used to compress the image data.
However, such apparatuses have a problem that they cannot successfully separate an area of an original document image that contains characters superimposed on a light photographic image from the photographic image and therefore compress the characters as the photograph area, thereby degrading the visibility of the characters.
In summary, the prior art has the following problems:
An enormous amount of data is generated when a color document image optically read by an image reader is digitized to produce digital image data. The size of the image data may be so large that it is unmanageable when it is stored or transmitted over a network.
To solve the problem, apparatuses have been devised that use compression technologies such as JPEG compression to compress image data in color document images to reduce its size. However, these apparatuses have problems that the visibility or the compression ratio of some objects (characters and photographs) contained in the color document images is degraded.
To solve these problems, apparatuses have been devised that separate a region containing characters from an area containing a photograph in a color document image and different compression technologies suitable for the different regions are used to compress the image data, thereby achieving high compression ratios without degrading the visibility of characters.
However, these apparatuses have problems that they fail to separate a character region from a photograph region when characters are superimposed on the photograph, thereby decreasing the visibility of the characters or the compression ratio of the image.